Both high efficiencies and high linearities can be achieved in RF amplifiers using a set of techniques known as amplitude reconstruction. In amplitude reconstruction, the amplitude information of a signal is removed, and the remaining phase-modulated signal is amplified using a highly efficient nonlinear amplifier. After amplification, the amplitude information is somehow returned to the signal.
One such technique for amplitude reconstruction is LINC (LInear amplification with Nonlinear Components), also referred to in older literature as outphasing. In this technique, the amplitude information in the signal is converted into phase modulation for two different signals. The phase modulation is performed in such a manner that when the two signals are amplified and then recombined, the resulting signal has the desired output amplitude. If the input signal has zero amplitude, then the two amplified signals will be 180 degrees out-of-phase and will cancel when recombined. If the input signal is at maximum amplitude, then the two amplified signals will be in-phase and will combine perfectly.
FIG. 1 is a block diagram of a LINC system 100 of the prior art. LINC system 100 comprises LINC modulator 102, power amplifiers PA1 and PA2, and combiner 104. The input signal to LINC system 100 is an amplitude-modulated carrier represented as A sin(ωt+φ). LINC modulator 102 generates two signals with phases φ+cos−1(A) and φ−cos−1(A). These two signals are then amplified by amplifiers PA1 and PA2, respectively, and combined by combiner 104 to produce an amplified replica γ of the input signal. Peak output is obtained when the two amplifiers add in-phase; zero output is obtained when they add out-of-phase. Intermediate phase values produce intermediate amplitudes.
In phasor notation, the input signal may be written as in Equation (1) as follows:u=Aejφ.  (1)The outputs of amplifiers PA1 and PA2 may be written as in Equations (2) and (3) as follows:V1=Ge+j(φ−cos−1A)  (2)andV2=Ge+j(φ+cos−1A),  (3)where G is the gain of both power amplifier PA1 and power amplifier PA2. The output γ of combiner 104 may be written as in Equation (4) as follows:γ=2GAejφ.  (4)There are two common methods for combining the two amplified signals generated by amplifiers PA1 and PA2. These two methods are described below in the context of FIGS. 2 and 3.
FIG. 2 shows a block diagram of a LINC system 200 of the prior art that employs a first method for combining the amplified signals generated by two power amplifiers. LINC system 200 has a LINC modulator and two power amplifiers that are analogous to those in LINC system 100 of FIG. 1. In LINC system 200, combiner 104 of FIG. 1 is implemented using a four-port hybrid combiner 204, also known as a power combiner. Combiner 204 receives the amplified signals from amplifiers PA1 and PA2 as two inputs and generates the sum and difference of the two signals as its two outputs. The sum is an amplified version of the input signal to the LINC system, while the difference signal is wasted in a dummy load. The advantage to such a technique is that each amplifier sees a perfectly matched load. However, some power is wasted in the dummy load, resulting in a loss of efficiency. (Note that, at zero input amplitude, all power is wasted in the difference port.)
FIG. 3 shows a block diagram of a LINC system 300 of the prior art that employs a second method for combining the amplified signals generated by two power amplifiers. Like LINC system 200, LINC system 300 has a LINC modulator and two power amplifiers that are analogous to those in LINC system 100 of FIG. 1. In LINC system 300, combiner 104 of FIG. 1 is implemented using a three-port, lossless combiner 304. Combiner 304 is implemented using a transmission line tee 306 with transmission line stubs (e.g., shunt reactances) 308 and 310 for impedance matching. Alternatively, combiner 304 may be implemented using a transformer. In either case, this LINC system has the advantage of efficiency over the four-port hybrid technique of LINC system 200, since no power is lost in the combiner. Unfortunately, the amplifiers no longer see perfectly matched loads at all output amplitudes. As a result, while the combiner itself is extremely efficient, most amplifiers that are used in such systems lose efficiency when connected to mismatched loads. In addition, their power outputs and phases may vary with the output amplitude of the system.
LINC system 300 uses shunt reactances (jBS and −jBS in FIG. 3) to improve the efficiency of a basic three-port system to improve amplifier matching at output amplitudes other than the maximum amplitude. In particular, shunts 308 and 310 are preferably placed at the electrical equivalent of one-quarter wavelength away (e.g., via quarter-wave delays 312 and 314) from combiner tee 306, where the shunts improve the load matching at a variety of output amplitudes. This greatly increases efficiency and linearity at some output amplitudes at the expense of some efficiency and linearity in other amplitudes. The optimum compensation depends heavily on the peak-to-average ratio of the signal to be amplified.